The invention relates to a liquid fuel injection system for a direct-injection engine.
A fuel delivery system for an internal combustion engine operating with a diesel cycle includes an engine-driven fuel injection pump with a plunger that reciprocates in a plunger cylinder to effect fuel delivery to each of the working cylinders of the engine. The pump stroke frequency is directly proportional to engine speed. A fuel control valve under the control of an electromagnetic solenoid actuator establishes controlled fuel delivery from the pump to fuel injection nozzles. In the case of a direct injection compression ignition engine, a fuel injection nozzle would be located in the combustion chamber of each of the engine cylinders. The solenoid actuator for the valve is responsive to controlled current pulses in the driver circuit of a digital electronic engine controller, whereby fuel is metered from the injector pump to the nozzles as the pump creates the necessary pressure pulses.
The plunger typically is driven by the engine camshaft, which operates the intake and exhaust valves of the engine. It is located in the cylinder head for the engine where it is exposed to engine lubrication oil. Any fuel that leaks past the clearance between the plunger and the plunger cylinder tends to commingle with the engine lubrication oil, thereby creating a dilution problem after an extended operating period.
It is possible to reduce leakage past the plunger by reducing the dimensional clearance between the plunger and the plunger cylinder. A reduction in the dimensional tolerance, however, increases the risk of pump seizure. Greater mechanical friction losses and increased wear, particularly in those applications in which the fuel temperature varies throughout a relatively wide temperature range, also may result from reduced clearance. Further, the machining required for a close tolerance fit between the plunger and the cylinder would increase manufacturing costs, which would make the pump and fuel supply system impractical for high volume manufacturing operations.
It also may be possible to reduce oil dilution due to fuel leakage past the plunger by increasing the length of the plunger, thereby increasing the leak flow path length from the high pressure pumping chamber to the engine camshaft cavity. This would result, however, in only a moderate decrease in leakage and would require a significant increase in the overall dimensions of the pump and control valve assembly. This would make it impractical for some commercial engine applications because of packaging constraints.
Examples of prior art pump and control valve assemblies for diesel engines of the kind that are commercially available may be seen by referring to U.S. Pat. No. 6,019,091. Further, copending application Ser. No. 09/272,021, filed Mar. 18, 1999, discloses a fuel pump and control valve assembly with elements corresponding to elements included in the present invention. The ""091 patent and the copending patent application are assigned to the assignee of the present invention.
It is an objective of the invention to reduce engine oil dilution with engine fuel by decreasing the leakage of fuel past the injection pump plunger into the lubrication oil circuit of the engine. In carrying out that objective, the pump and fuel control valve assembly of the invention comprises a fuel pump body with a pump cylinder for receiving a reciprocating pump plunger. A pump plunger spring normally urges the plunger to a retracted position. The plunger is driven during its working stroke by the engine camshaft, which is driven at one-half engine crankshaft speed.
The plunger and the cylinder define a high pressure working chamber that communicates with an injection nozzle through a high pressure fuel delivery passage, which is intersected by a pump flow control valve. Fuel is supplied to the control valve and to the working chamber of the pump from a fuel supply pump. The control valve opens and closes the fuel flow through the high pressure fuel delivery passage in accordance with commands transmitted to a solenoid actuator by an engine controller module. The valve is opened and closed in timed relationship with respect to the stroking of the plunger so that an initial pilot pulse is delivered by the nozzle to the engine combustion chamber. This is followed, in turn, by a main fuel delivery pressure pulse at the outset of the compression stroke of the engine cylinder.
The pump and control valve assembly of the invention comprises a pump housing or body with a pumping chamber defined by a cylinder in a cylinder body. A plunger is situated in the cylinder to define a high pressure fuel pump cavity, which communicates with the fuel injector nozzle. In one embodiment of the invention, the plunger and the cylinder are located in a common valve body or housing. In another embodiment of the invention, the cylinder is situated in a first pump housing, and the control valve assembly is situated in a separate valve housing, the two housings being joined by a housing portion in which are situated crossover fuel flow passages. In each instance, a single supply and return fuel passage extends to the pump and control valve assembly from a fuel pump. For this reason, the design commonly is referred to as a monorail design. Flow passages for the fuel to and from the fuel supply pump are not defined by separate supply and return passages as in a dual rail arrangement.
The pump plunger displaces fuel in the pump cavity as fuel is delivered by the high pressure fuel delivery passage to the injector nozzle.
At least one low pressure leak-off passage communicates with one or more fuel leak ports formed in the pump housing. A leak flow path in the passage defined by a predetermined clearance between the plunger and the plunger cylinder communicates with the low pressure leak-off passage, whereby fuel flow that leaks past the plunger is returned to the fuel reservoir for the fuel supply pump rather than flowing to the region of the camshaft in the engine housing. The pump plunger, when it is moved to a retracted position, covers the leak ports. Thus, the fuel circuit is independent of the lubrication oil for the engine so that oil dilution is eliminated or substantially reduced. This characteristic increases the durability of the fuel injection pump and control valve assembly and reduces maintenance costs for the engine.
In a typical operating environment for the engine, the fuel supply and return passage may be pressurized at a value of about 5 bar, whereas the low pressure leak-off passage that communicates with the fuel reservoir for the fuel supply pump may be at a substantially lower value, such as 1 bar. This pressure differential makes it possible for a leakage flow path through the clearance between the plunger and the cylinder to be diverted to the low pressure leak-off passage rather than to the camshaft region of the engine.